Versatility of hydrophilic and antifouling PVDF ultrafiltration membranes tailored with polyhexanide coated copper oxide nanoparticles

Journal Pre-proof Versatility of hydrophilic and antifouling PVDF ultrafiltration membranes tailored with polyhexanide coated copper oxide nanoparticles Meenakshi Sundaram Sri Abirami Saraswathi, Dipak Rana, Kumar Divya, Shanmugaraj Gowrishankar, Alagumalai Nagendran PII:

S0142-9418(19)32195-6

DOI:

https://doi.org/10.1016/j.polymertesting.2020.106367

Reference:

POTE 106367

To appear in:

Polymer Testing

Received Date: 25 November 2019 Revised Date:

2 January 2020

Accepted Date: 17 January 2020

Please cite this article as: M.S. Sri Abirami Saraswathi, D. Rana, K. Divya, S. Gowrishankar, A. Nagendran, Versatility of hydrophilic and antifouling PVDF ultrafiltration membranes tailored with polyhexanide coated copper oxide nanoparticles, Polymer Testing (2020), doi: https://doi.org/10.1016/ j.polymertesting.2020.106367. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Versatility of hydrophilic and antifouling PVDF ultrafiltration membranes tailored with polyhexanide coated copper oxide nanoparticles

Meenakshi Sundaram Sri Abirami Saraswathia, Dipak Ranab, Kumar Divyaa, Shanmugaraj Gowrishankarc, Alagumalai Nagendrana,*

a

Polymeric Materials Research Lab, PG & Research Department of Chemistry, Alagappa

Government Arts College, Karaikudi - 630 003, India b

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur

St., Ottawa, ON, K1N 6N5, Canada c

Department of Biotechnology, Science Campus, Alagappa University, Karaikudi - 630 003,

India

*Corresponding author: E-mail: [email protected](A.Nagendran); Tel.: 91-4565-224283; Fax: 91-456527497 1

Abstract Hydrophilic poly(vinylidene fluoride) (PVDF) nanocomposite ultrafiltration (UF) membranes with excellent antifouling and antibiofouling characteristics are fabricated by employing polyhexanide coated copper oxide nanoparticles (P-CuO NPs). The presence of PCuO NPs is played a significant role in altering the PVDF membrane matrix and probed by XRD, FTIR, FESEM and contact angle analysis. The PVDF/P-CuO nanocomposite membranes exhibited an outstanding antifouling performance indicated by the superior pure water flux, effective foulant separation and maximum flux recovery ratio during UF experiments as a result of the formation of the hydrophilic and more porous membrane due to the uniform distribution of P-CuO NPs. Particularly, the PVDF/P-CuO-3 membrane showed higher PWF of 152.5 ± 2.4 lm-2h-1 and porosity of 64.5 % whereas the lower contact angle of 52.5°. Further, it showed the higher rejection of 99.5 and 98.4 % and the flux recovery ratio of 99.5 and 98.5 % respectively for BSA and HA foulants, demonstrated its increased water permeation, foulant separation and antifouling behavior. Further, the decent antibacterial activity is showed by the PVDF/P-CuO nanocomposite membranes with the formation of halo-zone around the membrane when exposed to the bacterial medium demonstrated that, by this process an antibacterial water treatment membrane can be developed by simple phase inversion technique with good membrane stability.

Keywords:

Poly(vinylidene fluoride); Polyhexanide coated copper oxide; Ultrafiltration membrane; Antibiofouling; Hydrophilicity

2

1. Introduction The broad range of applications of polymeric membranes in wastewater treatment is ubiquitous [1]. Nevertheless, the deposition of organic and inorganic compounds in the feed on the surface as well as inside the pores causes longstanding biofouling and organic fouling that limits the membrane performance in industrial fields. Generally, during the filtration the materials such as dust particles, suspended solids, proteins, microorganisms, etc., considerably cause undesirable damage by their accumulation on the membrane surface [2]. The fouling happens in an eventual way in which concentration polarization is the primary factor where the deposition of foulants in the vicinity of the membranes due to the applied operational pressure. The rising concentration of foulants on the membrane surface over time is more pronounced which often leads to serious membrane damage is known as fouling. As a negative consequence of this, during filtration experiments, water flux decline with poor foulant separation performance is noticed [3,4]. The persistence of these contaminants in water from ineffective separation technology negatively affects its quality and makes it inappropriate to use and will become a threat to living organisms and to the environment. The modification of hydrophobic membranes into hydrophilic can cause a noticeable improvement not only in water permeation but also in membrane antifouling properties. Another important bottleneck is the phenomenon of biofouling which is considered to be nearly irreversible as the adhered bacteria can grow and reproduce which often leads to the formation of undesirable biofilm that over time will cover the entire membrane surface thereby affects the overall membrane performance [5]. Therefore, novel and effective methods to prevent pathogenic bacterial activity on the membranes are needed. Recently, a strategy of the addition of antibacterial or antiseptic compounds to the membrane matrix has been proposed by the researchers to effectively prevent or mitigate the damage of membrane by bacteria [6]. Among the various kinds of antibacterial compounds, polyhexanide or poly(hexamethylenebiguanide) (PHMB) is a cationic biocide and widely 3

employed in the medicinal and pharmaceutical industries due to its long-lasting antibacterial activity, low toxicity, chemical stability, etc.[7]. Küsters et al. reported that the PHMB binds to the negatively charged phosphate head groups of phospholipids at the bacteria cell wall, causing increased rigidity, sinking non-polar segments into hydrophobic domains, disrupting the membrane with subsequent cytoplasmic shedding culminating in cell death [8]. US Environmental Protection Agency (EPA) has been reviewed the risk of adverse health effects of PHMB to the living things and found that very low [9]. Being a cationic biocide PHMB is able to form a stable cation layer on the surface of the substrate which can act as a barrier for the microorganisms thereby prevents microbial membrane damage [10]. Besides, the high water permeability is an important criterion for a good membrane material. The addition of pore formers, hydrophilic additives or coatings could alter the membrane morphology and roughness; thereby enhance the water permeation properties in a favorable way [11]. Off-late, the application of nanotechnology is increasing in membranebased water treatment as these nano-sized particles own some interesting size-dependent characteristics such as high reactivity, large surface to volume ratio, good thermal resistance and high hydrophilicity that are different from the concern bulk material [12, 13]. Vantanpour et al. studied the different sizes of TiO2 nanoparticles (NPs) in altering the poly(ether sulfone) (PES) membrane and reported that the TiO2 with the smallest size has a positive influence where the comparatively bigger ones are diminished the membrane performance due to the particle agglomeration [14]. The main aim of NPs incorporation into the membrane is to impart advantageous morphology (caused by the interaction between the base polymer chain and nanoparticle interface), hydrophilicity and to control membrane fouling and biofouling properties [15]. Even though, the silver-containing NPs are one of the most effective classes of nanomaterials which has both high hydrophilicity and biocidal activity, its relatively expensive cost gives rise to a more budget-friendly nanoadditive. In this regard, the copper is one such an alternate 4

nanoadditive which can effectively mimic the characteristics of the silver NPs with nearly 1 % cost of silver. Wang et al. reported that the Cu2O NPs have the preferred properties such as hydrophilicity and intrinsic antimicrobial to be employed as effective nanofiller [16]. Krishnamurthy et al. reported that the humic acid and oil removal efficiency of PES membranes has been tremendously enhanced with the copper oxide NPs due to the excellent hydrophilicity and higher antifouling property of the resultant PES nanocomposite membrane [17]. Herein, novel polyhexanide coated CuO NPs (P-CuO) incorporated PVDF membranes is synthesized to alleviate the organic fouling as well as biofouling and to increase the water permeation performance. Hence in this study, we made an attempt to minimize the issue of nanoparticle agglomeration with a cationic polymeric coating which imparts a positive charge to P-CuO nanoparticle and enhances the nanoparticle dispersion in the nanocomposite membrane matrix due to the electrostatic repulsion between the P-CuO particles. This work involves the (i) Synthesis of CuO NPs by an aqueous precipitation method, (ii) Coating of polyhexanide on as-synthesized CuO NPs (iii) Fabrication and characterization of P-CuO incorporated PVDF nanocomposite membranes (iv) Performance evaluation of bare and tailored nanocomposite membranes by the various experimental methods including water flux measurement, foulant rejection capability, antibiofouling efficiency against gram-negative and gram-positive bacteria. 2. Materials and methods 2.1 Synthesis of copper oxide nanoparticles All reagents and chemicals received are of analytical grade and used without any further purification. Copper nitrate and poly(vinyl pyrrolidone) (PVP) are obtained from Sigma-Aldrich, USA. Copper oxide nanoparticles are prepared by a facile aqueous precipitation method as reported previously[18]. The pre-determined quantity of Cu(NO3)2 and PVP are mixed in 100 ml de-ionized water is heated and stirred well for 2 h. When the 5

temperature reaches 60ºC, 1M NaOH is added drop-wise into the above mixture. The process of heating is continued till a black precipitate is obtained, which is then centrifuged and dried at 50ºC in a hot-air oven for 1 h. 2.2 Coating of polyhexanide on CuO NPs The coating on NPs is basically a kind of functionalization which offers ordered secondary layer or structure with novel properties [19]. Herein, polyhexanide is employed as an antibacterial grafting material (Figure 1), which is a member of the cationic polymeric guanidine family to impart antibacterial efficiency to the CuO NPs. After that, the synthesized CuO NPs and polyhexanide are mixed at an equal ratio in water to achieve grafting. Subsequently, the mixture is stirred well for 2 h at 20ºC to obtain P-CuO NPs.

Figure 1 Chemical structure of PHMB 2.3 Morphological and functional group characteristics The chemical composition of P-CuO NPs is probed by FTIR using RX1–PerkinElmer spectrophotometer in the range of 400- 4000 cm-1. Further, the morphological properties of PCuO such as shape, size are probed by FESEM, Carl Zeiss, Germany. 2.4 Fabrication of neat PVDF and PVDF/P-CuO membranes Neat PVDF membranes are fabricated on a clean glass plate with a thickness of 200 µm using a doctor’s blade by phase inversion technique [20]. The casted membrane is subsequently submerged into the gelation bath (contains 2 L of DI water, 2.5% solvent and 0.2 g surfactant) for 1 h, after which prepared membranes were stored in deionized water for 24 h for 1 h. Thereafter, the membranes are cleaned by de-ionized water and stored in a 1 % 6

formalin solution. To fabricate customized PVDF/P-CuO nanocomposite membranes prior to the addition of PVDF, P-CuO NPs at different additive concentrations of 0.0, 0.5, 1, 2 and 3 wt.% are dispersed in NMP individually via ultra-sonication for 30 min to achieve uniform distribution of nanoparticles throughout the PVDF membrane matrix. 2.5 Characterization of neat PVDF and PVDF/P-CuO membranes 2.5.1 XRD X-ray diffraction pattern of all the membranes is probed by PAN analytical Advances Bragg-Brentano X-ray diffractometer equipped with monochromatized high-intensity Cu Kα radiation with a scan rate of 0.06º per sec. in 2θ scale between 10 and 80º. 2.5.2 Ultrafiltration performance A lab-scale dead-end UF kit with a capacity of 300 ml is employed to assess the membrane’s UF performance [20]. The membranes to be tested are cut into desired sized coupons with the active membrane area of 38.5 cm2 fitted into the holder with the help of an “O” ring. All the membranes are compacted for 3 to 4 h at 345 kPa to attain the steady-state and then the actual water permeation performance ( J

J

=

) is measured by Eqn. (1):

V (1) A∆t

Here ‘V’ denotes the volume of pure water (L), ‘A’ denotes the active surface area of the membrane (m2) and ‘t’ represents the permeation time (h). To determine their separation and antifouling capacity of the membranes, bovine serum albumin (BSA) and humic acid (HA) are prepared at pH 7.2 and employed as model foulants at a concentration of 1000 ppm each. Permeate from each membrane is collected and analyzed with Systronics, 2201 UV–visible double beam spectrophotometer at 280 nm using Eqn.(2) [21]. Subsequently ,the membranes are backwashed with de-ionized water for 1 h and again the pure water flux ( J

) is measured. Afterward, the antifouling property of the 7

membranes isevaluated by their percentage water flux recovery from the ratio of J J

and

using Eqn. (3). High fouling resistance membranes do not show much more difference

between J

and J

% Solute rejection = 1 –

C (2) C

Here Cp is the permeate concentration and Cf is the feed concentration (mg/L). J % Water flux recovery = & J

' (3)

2.5.3 Hydrophilicity and porosity The hydrophilic character of the membranes is determined by their water-absorbing capacity, which is evaluated by the water contact angle (WCA) and equilibrium water content (EWC). 1 µL of water is placed on the membrane top surface and the WCA of the membranes is measured using a contact angle goniometer, VCA Optima surface analyzing system. To determine the EWC of the membrane using Eqn. (4), it is immersed in de-ionized water for 24 h and its wet weight is noted (Mw), then it is dried in the hot-air oven at 40ºC, weighed and its dry weight (Md) is noted. These weight measurements are further employed in assessing the percentage porosity by dividing the volume of the pores by the total volume of the membrane using Eqn. (5). To attain more accurate results, all the experiments are repeated two or more times for each membrane and the values are averaged. [20, 22] % EWC =

M − M, × 100 (4) M

% Porosity = 2

46

2 3 2

45 2 3 2

+

45

8

× 100 (5)

Where ρ



and ρ represents the water density and the membrane density (in gm/cm3)

respectively. 2.5.4 Average pore radius The mean pore radius is an approximation of membrane true pore size throughout the matrix [23]. The average pore radius can be calculated by using the Guerout–Elford–Ferry equation as follows: (2.9 − 1.75ɛ)8ηhQ r: = ; (6) ɛ A ∆P Where η is the water viscosity (8.9 × 10-4 Pa s), Q is the quantity of permeate per unit

time (lm-2h-1), ∆P is the operating pressure (345 kPa), h is the membrane thickness (m), A is the membrane active surface area (cm2) and ɛ is the porosity of the membranes. 2.5.5 Morphological analysis The top surface and cross-sectional morphology of the membranes are analyzed by high-resolution FESEM (Carl Zeiss, Germany) to probe their pore structure and nanoparticle distribution. 2.5.6 Antibiofouling activity The antibacterial efficiency of the membranes against gram-negative and grampositive bacteria is evaluated by halo-zone method. The test bacteria are cultured separately and diluted in nutrient broth (NB) medium using a similar method discussed in our previous work [24]. The NB medium is then spread on separate Petri-dishes along with nutrient agar culture. Neat PVDF and PVDF-3 membranes are chosen to probe and compare the antibiofouling effect of the incorporation of P-CuO NPs. Membranes are cut into small circular sections and placed on the culture plate. After 24 h, the anti-bacterial property is evaluated by measuring the width of the halo-zone around each membrane.

9

2.5.7 Stability The stability of the P-CuO NPs in the PVDF membrane matrix is investigated by analyzing its release permeates at different time intervals using an atomic absorption spectrophotometer (AAS, Shimadzu, Model-AA 6300, Japan). 3. Results and discussion 3.1FT-IR and FESEM of CuO and P-CuO NPs

Figure 2. FTIR spectra of CuO and P-CuO nanoparticles.

Figure 2 depicts the FTIR spectra of raw and polyhexanide coated P-CuO NPs. The bands obtained at 2781 and 3581 cm-1 for raw CuO correspond to the symmetric and asymmetric stretching vibration of the O–H bond respectively. The bands observed at 460, 524 and 1616 cm-1corresponds to the characteristic stretching vibrations of Cu‒O bond and the successful formation of copper oxide nanoparticle is confirmed [25]. In the case of PCuO, the band at 860 cm-1is ascribed to the N-H stretching vibration of C-N-H or C=N-H and the band at 1082 cm-1 corresponds to the C=N stretching vibration are confirmed the successful coating of polyhexanide on CuO nanoparticle [26]. Further, the C=N stretching 10

band is supposed to appear around 1625 cm-1 is absent, which may be overlapped by the strong absorption band of Cu-O stretching vibration.

Figure 3. FESEM images of CuO (a,b) and P-CuO (c,d) nanoparticles at different magnifications. Figure 3 depicts the FESEM images of CuO and P-CuO NPs with different magnifications (5 and 20µm). It can be obviously seen that the raw CuO NPs are clumsy and agglomerated whereas the P-CuO NPs are appeared to be more uniform with a definite shape. The smoother surface of P-CuO NPs indicates the uniform coating of polyhexanide which is useful in the improvement of the dispersion of NPs in solvent possible due to the strong electrostatic repulsion. The dispersion stability of P-CuO NPs is observed better than CuO NPs, however, the size of particles is in nano-grade with150–300 nm in diameter.

11

3.2 Characterization of PVDF and PVDF/P-CuO membranes 3.2.1 XRD pattern and FESEM Figure4 showed the XRD patterns of PVDF and PVDF/P-CuO membranes.The two peaks at 18.5° and 20.5° in the pristine PVDF membrane were associated with (200) and (100) and it matches with the previously reported XRD pattern of pristine PVDF [20]. Further, the sharp peaks obtained in the XRD pattern of PVDF/P-CuO nanocomposite membranes at the 2θ angles of 32.3, 35.5 and 39.3 correspond to (110), (002) and (200) planes, respectively which confirms the monoclinic crystalline structure of the NPs [27]. Moreover, the position and the relative intensity of the diffraction peaks perfectly matched with the standard JCPDF card No. 05-0661 and 78-1969 further confirms the successful incorporation of P-CuO NPs in the PVDF membrane matrix. The ‘d’ spacing of 2 to 2.3 Å in the diffraction pattern of P-CuO NPs is attributed to the presence of amorphous polyhexanide on the CuO NP surfaces [28, 29].

Figure 4. XRD of PVDF and PVDF/P-CuO nanocomposite membranes. 12

Figure 5.Surface FESEM images of neat and nanocomposite PVDF membranes.

Figure 6.Cross sectional FESEM images of neat and nanocomposite PVDF membranes.

FESEM images revealed that PVDF and PVDF/P-CuO nanocomposite membranes possess compact porous structure in the surface with narrowly formed voids in the bulk phase. As shown in Figures 5 and 6, the pristine PVDF membrane possesses less-pores with the minimal number of non-connected macrovoids and rigid/compact polymeric structure possibly due to its high hydrophobicity. Hence restricts the formation of pores and interconnected voids thereby confine the water permeation performance of the membrane. On 13

the other hand, the PVDF/P-CuO nanocomposite membranes showed a much porous top surface with comparatively interconnected macrovoids in the bulk which seems to be improved with the increasing concentration of P-CuO in the membrane matrix. Moreover, the P-CuO nanoparticles can be obviously seen on the surface of the nanocomposite membranes as bright spots indicating their successful incorporation into the base PVDF membrane. This kind of morphological feature supposed to promote membrane water permeation properties and separation performances [30]. Thanks to the highly hydrophilic CuO NPs and polyhexanide coating on it which is very much helpful in improving the nanoparticle dispersion throughout the membrane matrix by imparting a positive charge to P-CuO. Generally, the thermodynamic properties of the solvent and non-solvent employed in a system have a great influence on altering the membrane morphology. Since the NMP has a high affinity towards non-solvent and polymer the solvent and non-solvent demixing towards phase inversion is delayed as a consequence of which the surface and cross-sectional morphology of pristine PVDF is irregular. However, in nanocomposite membranes, the incorporated P-CuO NPs played a dominant role which increases the rate of de-mixing during phase inversion. This favorable phenomenon is attributed to the high affinity of P-CuO NPs towards the water which is drawn mainly due to the superior hydrophilic nature of CuO. Also, it can be seen that at higher P-CuO concentrations number of NPs are scattered in the surface of the nanocomposite membrane without particle agglomeration which is attributed to the polyhexanide coating on CuO that keeps the NPs away from one another due to the electrostatic repulsion between them. Hence the most important bottleneck of using NPs as hydrophilic additives has been removed by employing polyhexanide coating. This approach is effectively altered the membrane morphology and showed that the nanocomposite membranes had improved hydrophilic properties which would be effective in the water permeation and foulant separation performances.

14

Table 1. Blend composition and equilibrium water content of neat and nanocomposite PVDF membranes. Blend composition (wt. %) PVDF

P-CuO

NMP

EWC (%)

Neat PVDF

17.5

0

82.5

51.8 ± 0.4

PVDF/P-CuO-0.5

17

0.5

82.5

55.2 ± 1.2

PVDF/P-CuO-1

16.5

1

82.5

63.4 ± 1.0

PVDF/P-CuO-2

15.5

2

82.5

72.2 ± 0.8

PVDF/P-CuO-3

14.5

3

82.5

81.5 ± 1.2

Membrane Code

3.2.2 PWF, EWC, contact angle and pore characteristics It is evidently seen from Figure 7 that the substantial improvement in the pure water permeation performance of nanocomposite membranes. Further, it clearly indicates that even at a very low concentration of P-CuO has a considerable influence in enhancing the water permeation properties of membranes. The increment in water permeability from 45.2 to 152.5 lm-2h-1 is attributed to two possible phenomena: (i) the surface/morphological properties of the modified membrane and (ii) high hydrophilicity of the incorporated P-CuO nanoadditive. As discussed in section 3.2.1, uniformly distributed P-CuO throughout the upper skin layer of the PVDF nanocomposite membranes with highly interconnected macrovoids facilitate much easier water permeation through membranes. Further, the high affinity of P-CuO NPs towards water promotes adsorption of more number of water molecules by the nanocomposite membranes. These water molecules are assumed to be transported through the sponge-like bulk structure of the nanocomposite membrane thereby increases the overall water permeate flux. These above-obtained results are in good agreement with the water contact angle and EWC of the membranes as shown in Table 1 which are the most important measures in determining the membrane water-absorbing capacity and surface hydrophilicity. Hence, these favorable results also indicated that the eventual increment of membrane hydrophilicity properties with the increasing addition of P-CuO NPs. A similar result is observed by 15

Nasrollahi et al. when employing CuO/ZnO nanocomposites to improve the filtration performance of PES membrane [31]. However, beyond a certain nanoadditive concentration, the water permeation and solute rejection performance of the PES-CuO/ZnO nanocomposite reported being diminished due to the agglomeration of NPs.

Figure 7. PWF of neat and nanocomposite PVDF membranes as a function of P-CuO NPs concentration. Generally, it has been reported in the literature that the most predominant reason behind the improvement in porosity and pore size of the nanoparticle incorporated membranes is that the migration of nanoparticle in the direction of membrane’s surface during phase inversion which creates more number of pores and macrovoids in the membrane surface and bulk respectively [32].Once the agglomeration occurs in the membrane matrix, the same phenomenon takes place with a major change of formation of pores with an enlarged pore radius, as the agglomerate size is comparatively bigger than that of the nanoparticle. As a negative consequence of this, the overall solute rejection performance of the concern membrane decreases as the increased pore radius allows the contaminant particles to penetrate through the membrane [33]. However, in this study, the particle agglomeration issue even at relatively high additive concentrations has been prevented by 16

polyhexanide coating which prevents agglomeration of nanoparticles throughout the PVDF membrane matrix due to the electrostatic repulsion between positively charged P-CuO NPs. This is in good agreement with the observed and nearly unchanged surface pore radius and solute rejection data as shown in Table 2 and Figure 8 respectively.

Table 2. Contact angle, porosity and pore radius of neat and nanocomposite PVDF membranes. Membrane code

Contact angle (°)

Porosity (%)

Pore radius (nm)

Neat PVDF

81.2 ± 0.5

23.4 ± 0.3

13.2 ± 0.2

PVDF/P-CuO-0.5

76.4 ± 0.4

33.8 ± 0.4

13.5 ± 0.3

PVDF/P-CuO-1

67.3 ± 1.0

55.2 ± 0.8

13.9 ± 0.5

PVDF/P-CuO-2

59.4 ± 0.8

61.2 ± 1.1

14.2 ± 0.4

PVDF/P-CuO-3

52.5 ± 1.2

64.8 ± 1.3

14.7 ± 0.5

3.2.3 BSA/HA rejection and antifouling performance As depicted in Figure 8a, all the P-CuO incorporated membranes exhibited better solute rejection performance than the pristine membrane when employed in the separation of lab-made solutions of complex chemical structures such as BSA HA (contains phenolic and carboxylic groups). It is evidently seen from Figure 8a that the nanocomposite membranes with increasing P-CuO content showed improvement in foulant rejection performance from 84.5 to 99.5% and 82.4 to 98.4 % for BSA and HA respectively. This favorable result indicates the formation of thicker membrane structure with increasing nanoadditive concentration that facilitates high water permeation through its porous morphology which is also helpful in maintaining superior foulant separation performance due to the denser top surface structure which can be clearly seen in the cross-sectional FESEM images. Even though the unmodified PVDF membrane possesses more compact morphology with the minimal number of surface pores to show lesser foulant rejection capacity due to its high hydrophobicity which allows the accumulation of foulants on the membrane surface [32]. 17

This leads to the eventual surface pore blockage as well as irreversible membrane damage and thereby significantly reduces the water permeation as well as solute rejection performance of pristine PVDF.

Figure 8. BSA and HA rejection (a) and flux recovery ratio (b) of neat and nanocomposite PVDF membranes. Figure 8b represents the FRR of neat as well as nanocomposite PVDF membranes plotted as the function of P-CuO modification. Generally, the flux recovery capacity is considered to be one of the important measures to determine the antifouling properties of a membrane

since

the

maximum

flux

recovery

denotes

minimum/no

membrane

damage/fouling. It could be obviously seen from the figure that, all the P-CuO NPs incorporated nanocomposite membranes showed a better flux recovery than the pristine PVDF membrane, particularly, the PVDF/P-CuO-3 membrane with 3 wt.% P-CuO NPs had the highest FRR of 99.5 and 98.5 % for BSA and HA respectively, which denotes the significant influence of the added hydrophilic nanofiller in improving the membrane antifouling properties. In our previous work, we have investigated that the effect of manganese dioxide (MnO2) nanoparticles in altering the hydrophilicity and antifouling properties of the PVDF membrane and reported that MnO2 is helpful in enhancing the 18

membrane flux recovery [20]. Herein, CuO NPs are also demonstrated to be beneficial in the improvement of membrane antifouling performances. Further, the eventual increase in porosity from 23.4 to 64.8 % for PVDF and PVDF/PCuO-3 membranes respectively clearly denotes that the increased concentration of P-CuO NPs in the membrane matrix results in more porous membrane morphology. Such an enhancement in membrane percentage porosity may be attributed to the accelerated solvent – non-solvent exchange rate during phase inversion caused by the P-CuO NPs present in the membrane matrix [34]. In detail, the incorporated P-CuO NPs cause thermodynamic instability in the casting dope during membrane formation by accelerating the solvent outflux and water in-flux. This phenomenon leads to the instantaneous liquid-liquid demixing that increases the rate of coagulation thereby results in the formation of the more porous membrane. A similar kind of results has been reported by Ho et al. [35]. 3.2.4 Stability of P-CuO nanoparticles

Figure 9. Stability of the P-CuO NPs on the PVDF nanocomposite membranes as a function of filtration time. The stability of P-CuO NPs in the PVDF membrane matrix is determined by collecting the permeate water at different time intervals during UF experiments and analyzing the samples by AAS and the results are depicted as Figure 9. The results revealed that the 19

copper concentration in the permeate flux is very low than the maximum contaminant limit stipulated by WHO that is 1.3 mg/L [36]. Hence, it has been suggested by the AAS results that the P-CuO NPs exhibited good stability in the PVDF matrix with minimum leach out/release of copper thereby makes the membrane safe to employ in water treatment applications without any potential health hazards to the living organisms. 3.2.5 Antibiofouling capacity

Figure 10. The anti-bacterial performance results of neat PVDF and PVDF/P-CuO-3 membranes against E. coli and S. aureus. From the results of halo-zone test, it can be observed that the PVDF/P-CuO nanocomposite membrane possesses strong antibacterial activity as there is a formation of a wider zone around itas shown in Figure 10. Meanwhile, the neat PVDF membrane is completely covered by bacteria. Generally, the antibacterial activity of any nanomaterial is greatly depended upon two important factors: surface charge of the nanomaterial and the structural complexity of the bacteria employed [37]. When a cationic nanoparticle 20

incorporated membrane exposed to anionic bacterial medium the interaction between the nanoparticle and bacteria results in the bacterial cell damage that eventually leads to the formation of the halo-zone around the membrane. It is noteworthy that the nanocomposite membrane displayed a much wider zone against S. aureus (gram-positive bacteria) than E. coli (gram-negative bacteria). Similar results have been reported by Mukherjee et al. when employed chitosan-coated iron oxide NPs in modifying polyacrylonitrile (PAN) membranes [38]. The possible reason behind this kind of observation could be the morphological difference between the S. aureus and E.coli. As a gram-negative bacterium, E.coli contains an outer membrane layer which blocks the direct interaction of the adsorbent thereby prevents bacterial damage to some extent. However, in gram-positive bacteria, the absence of membrane protection layer facilitates much easier cell attack and that’s why the inhibitory zone appears to be much wider. 4. Conclusions Highly hydrophilic PVDF/P-CuO nanocomposite membranes are developed with excellent antifouling as well as antibiofouling activity via the phase inversion technique. The P-CuO NPs are distributed well throughout the PVDF membrane matrix, revealed by surface FESEM analysis. Thanks to the polyhexanide coating on CuO which prevents nanoparticle agglomeration by imparting a positive charge to CuO NPs. Further, the cross-sectional FESEM analysis clearly denotes the number of macrovoid formation in the membrane bulk which increases with increasing P-CuO concentration. This morphological change is very much beneficial in enhancing the water permeation performance evidenced by an increasing trend in PWF from pristine PVDF to PVDF/P-CuO-3 membrane. Of course, the enhanced percentage porosity of the nanocomposite membranes with nearly unchanged pore radius also gives raise to water-absorbing property due to which the water contact angle of the modified membranes is considerably decreased. Compared to the pristine membrane, the modified membranes showed better solute rejection, high antifouling property with maximum flux 21

recovery during UF experiments particularly due to the tremendous enhancement in the overall membrane hydrophilicity. Furthermore, the tailored membranes exhibited outstanding antibiofouling activity against gram-positive and gram-negative bacteria due to the combined activity of antibacterial polyhexanide coating and copper thereby limits the threat of biofouling in the membrane. This research disclosed that polyhexanide coated CuO NPs could be a potential inorganic additive to alter the PVDF UF membrane matrix to be employed in water treatment.

References 1. Geise, G.M., Lee, H.S., Miller, D.J., Freeman, B.D., McGrath, J.E. and Paul, D.R., 2010. Water purification by membranes: the role of polymer science. Journal of Polymer Science Part B: Polymer Physics, 48(15), pp.1685-1718. 2. Chen, G., Lin, Q., Chen, S. and Chen, X., 2016. In-plane biaxial ratcheting behavior of PVDF UF membrane. Polymer Testing, 50, pp.41-48. 3. Zhang, W., Luo, J., Ding, L. and Jaffrin, M.Y., 2015. A review on flux decline control strategies in pressure-driven membrane processes. Industrial & Engineering Chemistry Research, 54(11), pp.2843-2861. 4. Xiao, K., Wang, X., Huang, X., Waite, T.D. and Wen, X., 2011. Combined effect of membrane and foulant hydrophobicity and surface charge on adsorptive fouling during microfiltration. Journal of Membrane Science, 373(1-2), pp.140-151. 5. Pramila, J., Melbiah, J.B., Rana, D., Gandhi, N.N., Nagendran, A. and Mohan, D., 2018. Permeation characteristics of tailored poly (m-phenyleneisophthalamide) ultrafiltration membranes and probing its efficacy on bovine serum albumin separation. Polymer Testing 67, pp. 218-22.

22

6. Hou, S., Dong, X., Zhu, J., Zheng, J., Bi, W., Li, S. and Zhang, S., 2017. Preparation and characterization of an antibacterial ultrafiltration membrane with N-chloramine functional groups. Journal of Colloid and Interface Science, 496, pp.391-400. 7. Mulder, G.D., Cavorsi, J.P. and Lee, D.K., 2007. Polyhexamethylene Biguanide (PHMB): An addendum to current topical antimicrobials. Wounds, 19(7), pp.173-182. 8. Küsters, M., Beyer, S., Kutscher, S., Schlesinger, H. and Gerhartz, M., 2013. Rapid, simple and stability-indicating determination of polyhexamethylenebiguanide in liquid and gel-like dosage forms by liquid chromatography with diode-array detection. Journal of Pharmaceutical Analysis, 3(6), pp.408-414. 9. EPA (2005), United States Environmental Protection Agency, Washington, D.C. /http://www.epa.gov/ oppsrrd1/REDs/phmb_red.pdfS, 2005. 10. Chindera, K., Mahato, M., Sharma, A.K.,

Horsley, H., Kloc-Muniak, K.,

Kamaruzzaman, N.F., Kumar, S., McFarlane, A., Stach, J., Bentin, T. and Good, L., 2016. The antimicrobial polymer PHMB enters cells and selectively condenses bacterial chromosomes. Scientific Reports, 6, p.23121. 11. Yin, J. and Deng, B., 2015. Polymer-matrix nanocomposite membranes for water treatment. Journal of Membrane Science, 479, pp.256-275. 12. Sri Abirami Saraswathi, M., Rana, D., Beril Melbiah, J. S., Mohan, D. and Nagendran, A.2018. Effective removal of bovine serum albumin and humic acid contaminants using poly (amide imide) nanocomposite ultrafiltration membranes tailored with GO and MoS2 nanosheets. Materials Chemistry and Physics, 216, pp. 170-176. 13. Ng, L.Y., Mohammad, A.W., Leo, C.P. and Hilal, N., 2013. Polymeric membranes incorporated with metal/metal oxide nanoparticles: a comprehensive review. Desalination, 308, pp.15-33. 14. Vatanpour, V., Madaeni, S.S., Khataee, A.R., Salehi, E., Zinadini, S. and Monfared, H.A., 2012. TiO2 embedded mixed matrix PES nanocomposite membranes: Influence of 23

different sizes and types of nanoparticles on antifouling and performance. Desalination, 292, pp.19-29. 15. Pendergast, M.M. and Hoek, E.M., 2011. A review of water treatment membrane nanotechnologies. Energy & Environmental Science, 4(6), pp.1946-1971. 16. Wang, Y., Liu, L., Cai, Y., Chen, J. and Yao, J., 2013. Preparation and photocatalytic activity of cuprous oxide/carbon nanofibres composite films. Applied Surface Science, 270, pp.245-251. 17. Krishnamurthy, P.H., Yogarathinam, L.T., Gangasalam, A. and Ismail, A.F., 2016. Influence of copper oxide nanomaterials in a poly (ether sulfone) membrane for improved humic acid and oil–water separation. Journal of Applied Polymer Science, 133(36), 43873. 18. Mayekar, J., Dhar, V. and Radha, S., 2014. Synthesis of copper oxide nanoparticles using simple chemical route. International Journal of Scientific and Engineering Research, 5(10), pp.928-930. 19. Kar, M., Vijayakumar. P.S., Prasad, B.L.V., Gupta, S.S. 2010.Synthesis and characterization of poly-L-lysine-grafted silica nanoparticles synthesized via NCA polymerization and click chemistry. Langmuir, 26 (8), pp. 5772-5781. 20. Sri Abirami Saraswathi, M., Rana, D., Nagendran, A. and Alwarappan, S., 2018. Fabrication of anti-fouling PVDF nanocomposite membranes using manganese dioxide nanospheres with tailored morphology, hydrophilicity and permeation. New Journal of Chemistry 42 (19), pp. 15803-15810. 21. Kanagaraj, P., Nagendran, A., Rana, D., Matsuura, T., Neelakandan, S. and Malarvizhi, K., 2015. Effects of poly (vinyl pyrrolidone) on the permeation and fouling-resistance properties of poly (ether imide) ultra-filtration membranes. Industrial & Engineering Chemistry Research, 54 (17), pp. 4832-4838.

24

22. Das, C. and Gebru, K.A., 2017. Preparation and Characterization of CA−PEG−TiO2 Membranes: Effect of PEG and TiO2 on Morphology, Flux and Fouling Performance. Journal of Membrane Science and Research, 3(2), pp.90-101. 23. Cuperus, F.P. and Smolders, C.A., 1991. Characterization of UF membranes: membrane characteristics and characterization techniques. Advances in Colloid and Interface Science, 34, pp.135-173. 24. Sri Abirami Saraswathi, M., Rana, D., Alwarappan, S., Gowrishankar, S., Vijayakumar, P. and Nagendran, A., 2019. Polydopamine layered poly (ether imide) ultrafiltration membranes tailored with silver nanoparticles designed for better permeability, selectivity and antifouling. Journal of Industrial & Engineering Chemistry, 76, pp. 141-149. 25. Ethiraj, A.S. and Kang, D.J., 2012. Synthesis and characterization of CuO nanowires by a simple wet chemical method. Nanoscale Research Letters, 7(1), p.70. 26. Xie, W., Han, Y. and Tai, S., 2017. Biodiesel production using biguanide-functionalized hydroxyapatite-encapsulated-γ-Fe2O3 nanoparticles. Fuel, 210, pp.83-90. 27. Rashad, M., Rüsing, M., Berth, G., Lischka, K. and Pawlis, A., 2013.CuO and Co3O4 nanoparticles: Synthesis, characterizations, and Raman spectroscopy. Journal of Nanomaterials, 6,pp. 82. 28. JCPDS cards, No. 05-0661. 29. JCPDS cards, No. 78-1969 30. Xu, R., Wang, J., Chen, D., Yang, F., Kang, J., Xiang, M., Li, L. and Sheng, X., 2018. Preparation of pH-responsive asymmetric polysulfone ultrafiltration membranes with enhanced anti-fouling properties and performance by incorporating poly (2-ethyl-2oxazoline) additive. RSC Advances, 8 (72), pp.41270-41279. 31. Nasrollahi, N., Vatanpour, V., Aber, S. and Mahmoodi, N.M., 2018. Preparation and characterization of a novel polyethersulfone (PES) ultrafiltration membrane modified

25

with a CuO/ZnO nanocomposite to improve permeability and antifouling properties. Separation and Purification Technology, 192, pp.369-382. 32. Sri Abirami Saraswathi, M., Rana, D., Divya, K., Gowrishankar, S., Sakthivel, S., Alwarappan, S. and Nagendran, A., 2020. Highly permeable, antifouling and antibacterial poly (ether imide) membranes tailored with poly (hexamethylenebiguanide) coated copper oxide nanoparticles. Materials Chemistry and Physics, 240, 122224. 33. Monash, P. and Pugazhenthi, G., 2011. Effect of TiO2 addition on the fabrication of ceramic membrane supports: A study on the separation of oil droplets and bovine serum albumin (BSA) from its solution. Desalination, 279 (1-3), pp.104-114. 34. Lee, J., Chae, H. R., Won, Y. J., Lee, K., Lee, C. H., Lee, H. H., Kim, I. C. and Min Lee, J.,2013. Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment. Journal of Membrane Science, 448, pp. 223-230. 35. Ho, K.C., Teow, Y.H., Ang, W.L. and Mohammad, A.W., 2017. Novel GO/OMWCNTs mixed-matrix membrane with enhanced antifouling property for palm oil mill effluent treatment. Separation and Purification Technology, 177, pp.337-349. 36. WHO, Guidelines for Drinking Water Quality: Health Criteria and Other Supporting Information, World Health Organization, Geneva, Switzerland, 1993. 37. Wen, Y., Yao, F., Sun, F., Tan, Z., Tian, L., Xie, L. and Song, Q., 2015. Antibacterial action mode of quaternized carboxymethyl chitosan/poly (amidoamine) dendrimer core– shell nanoparticles against Escherichia coli correlated with molecular chain conformation. Materials Science and Engineering C, 48, pp.220-227. 38. Mukherjee, M. and De, S., 2017. Investigation of antifouling and disinfection potential of chitosan coated iron oxide-PAN hollow fiber membrane using Gram-positive and Gram-negative bacteria. Materials Science and Engineering C, 75, pp.133-148.

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Research Highlights •

Successful fabrication of PVDF/P-CuO nanocomposite UF membranes.

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PVDF/P-CuO membranes showed superior hydrophilicity and water permeation

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PVDF/P-CuO membranes exhibited higher BSA/HA rejection and flux recovery ratio

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PVDF/P-CuO membranes displayed both anti-organic fouling and anti-biofouling.

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PVDF/P-CuO membranes are promising for potential use in the water treatment.

Data will be made available on request.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.